1. Field of the Invention
The present invention relates to a DC-DC converter performing DC-DC (direct current-direct current) conversion by switching a switching element ON and OFF, and in particular to a DC-DC converter employing Pulse Frequency Modulation (hereinafter referred as PFM) control of a fixed ON time of the switching element.
2. Related Art
In recent years, in DC-DC converters used as a power supplies of portable devices and the like, reducing current consumption in a standby mode has been requested. PFM control of a fixed ON time is a control technique which adjusts an output voltage of the DC-DC converter by fixing an ON time of the switching element of the DC-DC converter and varying an OFF time of the switching element, and when there is little load current, the DC-DC converter operates in a discontinuous current mode and a switching frequency decreases in approximate proportion to decreasing of a load current. Therefore, switching loss can automatically be reduced under light loads such as in standby mode.
When the load current is somewhat larger, the DC-DC converter with PFM control operates in a continuous current mode, and operates at an approximately constant switching frequency determined depending on a ratio between the input voltage and the output voltage. Thus, when the DC-DC converter is used under the condition of the constant input voltage, the switching frequency under a heavy load can be adjusted in a desired range by selecting the ON time to be a suitable value.
However, in the case where the input voltage varies due to the drain of a battery or use of a plurality of power supplies, or in the case of use after the output voltage is set arbitrarily, because the switching frequency is varied by an operating condition, power conversion efficiency is reduced. As a countermeasure, it has been proposed, for example, in Japanese Laid-Open Patent No. 2004-96982 that PFM control is performed under light loads, and it is changed under heavy loads to Pulse Width Modulation (hereinafter referred as PWM) control which the switching frequency is fixed and the ON time and the OFF time are variable.
However, in the conventional DC-DC converter as described above, in the case where the switching control mode is changed between light-load times and heavy-load times, high performance can be maintained in a wide load region, but because current detection to change the control circuit is necessary and it is necessary to provide control circuits for two systems, problems arise in that the circuit configuration becomes complicated, and current consumption in the control circuits increases.
It is therefore an object of the present invention to provide a DC-DC converter in which reduction of current consumption can be obtained with a simple configuration and the switching frequency decreases in the light load case without being concerned with a change of the input voltage and the output voltage, and further, it is possible to operate at an approximately constant switching frequency in the heavy load case.